1. Field of the Invention
The present invention relates to an optical wavelength converter that is used in a communication field for converting one wavelength into another wavelength while ensuring continued performance of an optical signal to be transmitted.
2. Description of the Related Art
In a conventional optical wavelength converter that utilizes change in phase of a semiconductor optical amplifier (hereinafter, abbreviated to its acronym, SOA), SOAs are inserted to both branches of a Mach-Zehnder (hereinafter, abbreviated to its acronym MZ) interferometer. Input signal light is inputted to one of the branches, and direct current light is inputted to both the branches. The change in phase, which is induced in the SOA by the input signal light, causes modulation of output signal light of the MZ interferometer. The increase in induced emission caused by the input signal light reduces a carrier density. As a result, a refractive index increases due to a plasma effect, which causes the change in phase.
Then, in order to expand an input dynamic range of the optical wavelength converter, a signal amplifying SOA for amplifying the input signal light is arranged immediately after an input port to deal with variation in intensity of the input signal light.
However, when an SOA having the same structure as that of wavelength converting SOA inserted to the branch of the MZ interferometer is used as the signal amplifying SOA, a gain of the SOA is easy to be saturated because the signal amplifying SOA has an optical confinement coefficient of 0.6, which is an excessively large value. Therefore, there arises a problem in that a waveform is distorted in the case of, for example, weak input signal light at a high speed exceeding a speed of 40 Gbps.
In view of this, the optical wavelength converter is structured such that a ratio of a working area to an optical confinement coefficient in the signal amplifying SOA is larger than a ratio of a working area to an optical confinement coefficient in the wavelength converting SOA, whereby deformation of the input signal light is prevented (for example, refer to JP 10-319454 A).
However, there is a problem in that, when the SOAs are formed to have different working areas of working waveguides, the waveguides are difficult to be formed with high precision because the waveguides each have a width of, for example, about 0.5 μm.